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Show 200 ON DERMAL PLATES OF HOMOSTEUS. [Mar. 17, pairs, all directed both backwards and outwards. These markings become very feeble anteriorly, and still await explanation ; behind they are very suggestive of an impression of the anterior part of the spinal cord with its divergent nerves, but in front they appear rather as if corresponding to the septa between the myocommas of the muscular system. The brain itself would certainly have occupied a more anterior situation. Both the lateral occipitals are well shown from the visceral aspect in the fossil, and that of the left side is represented in fig. 2. The postero-external angle and posterior border are much thickened for articulation with the dorso-lateral plate, but the specimen is somewhat fractured at this margin. A narrow lenticular facette (f) extending along the greater part of the outer border, and a large distinct facette (f) occupying the anterior apex of the bone, show that it overlapped the marginal and central plates ; but the remainder of the visceral surface would be in direct contact with the soft parts of the animal, and it is quite smooth or exhibits only lines of growth, except at the postero-external angle, where a feeble short ridge is directed forwards and inwards, evidently to strengthen the bony lamina at that point. Though occurring on a detached fragment of matrix, the bone here interpreted as anterior median ventral (fig. 3) was found so closely associated with the occipitals already described, that Mr. Calder, who is an experienced collector, has no doubt as to its pertaining to the same individual. Its borders are completely preserved, thus proving that its obvious bilateral symmetry is not accidental ; and the absence of facettes on the smooth visceral aspect, except possibly behind, indicates that it must have been overlapped by any lateral plates that were originally adjoining. The anterior border is nearly straight, and accompanied by a low, rounded, transverse ridge ; and the anterior is the broader end of the bone, being produced on either side into a short and broad triangular extension. The thin hinder border is excavated by a symmetrical re-entering angle, into which the front margin of a diamond-shaped median ventral plate doubtless fitted. The fourth plate under consideration (fig. 4) also occurs upon a detached fragment of matrix, and is shown only as an impression of its visceral surface. The borders, however, are almost completely preserved ; and the impression proves that the bone was not quite flat, but marked by two broad rcunded folds extending and diverging from the shortest of the four margins. This margin is gently excavated, with a sharp angle at one end and a rounded corner at the other ; while the only distinct evidence of an overlapping facette is at the opposite border, which seems to have been much attenuated. The state of preservation does not permit of ascertaining whether any other facettes were present; but the outline of the bone agrees so closely with that of the anterior ventrolateral plates of Coccosteus that it is probable the two long borders joining those already noticed were also overlapping. Indeed, the element now described seems to differ only from the anterior ventro-lateral plates of Coccosteus in its |